1. Field of the Invention
This invention relates to a method for producing a diffraction grating, and more particularly, to a method for producing a phase-shifted diffraction grating such as .lambda./4-shifted diffraction grating for use in various devices such as filters and semiconductor lasers, typically distributed feedback (DFB) type lasers.
2. Related Background Art
Diffraction gratings have been used in opto-electronic fields as various optical circuit devices such as filters, optical couplers, distributed feedback (DFB) type lasers, distributed Bragg reflection (DBR) type lasers and the like. In particular, DFB and DBR lasers using diffraction gratings as resonators for lasers have been studied and developed for their stable dynamic single mode characteristics that make oscillation wavelengths stable even if the lasere are directly modulated.
On the other hand, .lambda./4-shifted DFB lasers with phase-shifted diffraction gratings have been developed several years ago to improve the oscillation singularity of these lasers (see Japanese Patent Laid-open No. 62-262004). The .lambda./4-shifted DFB laser oscillates in a single longitudinal mode which is equal to a Bragg wavelength thereof, so that a previous problem of oscillation occurring in two longitudinal modes in conventional lasers has almost been solved.
The following methods have been used in fabricating structures that achieve the .lambda./4-shift effect, when classified roughly.
(1) The period or phase of a diffraction grating is reversed halfway thereof, and its phase is shifted by .lambda./4 or quarter wave.
(2) The phase of light is equivalently shifted by partially adjusting its propagation constant by means of a change in the width of a diffraction grating and the like.
Method (1) can be realized comparatively readily. In the method (1), ordinary diffraction grating producing techniques can be adopted.
Further, the following methods are known to produce a phase-shifted type diffraction grating, roughly classified.
(1) An electron beam direct depicting method is shown in FIG. 1. An electron beam 1 is scanned in directions of arrows on a substrate 2 to form a diffraction grating 3.
(2) An interference exposure method in which both of high-resolution positive and negative photoresists of novolac series are used and an intermediate layer is provided to prevent mixing between these photoresists as shown in FIG. 2.
Initially, a 70 nm-thick negative photoresist (ONNR-20) 12, a 60 nm-thick cyclised polyisoprene (OBC) 13 and a 0.5 .mu.m-thick positive photoresist (MP-1400) of AZ layer 14 are spin-coated successively onto an InP substrate 11. Next, striped patterns of positive photoresist are generated by photolithography. Using these patterns as a mask, the cyclised polyisoprene 13 and the negative photoresist 12 are etched off by an H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 solution. Then, negative photoresist line patterns covered with the cyclised polyisoprene 13 are obtained by removing the positive photoresist 14 by means of its developer. After this, a positive AZ photoresist layer 15 for interference exposure and a 70 nm-thick polyvinyl alcohol (PVA) 16 are spin-coated. The PVA 16 enhances the photosensitivity of the negative photoresist 12 because oxygen in air does not penetrate this thin film. Using this technique, almost the same photosensitivity is obtained for the positive and negative photoresists 15 and 12. After holographic exposure on the photoresist patterns using an He-Cd laser beam (.lambda.=325 nm), the photoresists 12 and 15 are developed and the photoresist pattern is transcribed to the substrate 11 by wet etching, separately for each photoresist region. Thus, the phase-shifted diffraction grating is fabricated. In this connection, see M. OKAI et al, "NEW HIGH-RESOLUTION POSITIVE AND NEGATIVE PHOTORESIST METHOD FOR .lambda./4-SHIFTED DFB LASERS", Electron. Lett., Apr. 9, 1987, Vol. 23, No. 8, pp. 370-371.
(3) An interference exposure method using a contact mask 21 of quartz as a phase mask is shown in FIG. 3. The contact mask 21 is made of a transparent material whose thickness is varied according to its region. In FIG. 3, reference numeral 22 designates a substrate surface, reference numeral 23 designates interference fringes, reference numeral 24 designates a laser resonator, reference numeral 25 designates a transient area and reference numerals 26a and 26b designate laser light beams entering the contact mask 21 at incident angles .theta..sub.L and .theta..sub.R, respectively. In this connection, see M. SHIRASAKI et al, ".lambda./4 shifted DFB-LD corrugation formed by a novel spatial phase modulating mask", OQE 85-60, FUJITSU LABORATORIES LTD., pp. 57-64.
(4) An interference exposure method using a projected image of a phase mask or a phase plate 31 is shown in FIG. 4. In FIG. 4, reference numeral 32 designates a beam splitter, reference numeral 33 designates a spatial filter, reference numeral 34 designates a collimating lens, reference numeral 35 designates a plane mirror, reference numeral 36 designates a lens system having a common focal point and reference numeral 37 designates a sample surface. In this connection, see S. TSUJI et al, "phase-shift grating for single mode DFB lasers fabricated by phase projection type holography", Informal Papers of Electronics Communication Academy Semiconductor-Material Division Grand Meeting, 1985, p. 127.
(5) A method in which a conventional interference exposure method is used and a part is reversely transferred onto a substrate using SiN.sub.x layer formed by an electron cyclotron resonance-chemical vapor deposition (ECR-CVD) method is shown in FIG. 5. In FIG. 5, initially, a diluted positive photoresist (MP-1400) 41 is spin-coated onto an InP substrate 42. Then an ordinary holographic exposure with an He-Cd laser beam (325 nm) and development are performed to make a photoresist corrugation 41 with a period of 240 nm. The SiN.sub.x film 43 is deposited onto it by ECR-CVD at room temperature. After spin-coating a photoresist 44 again, striped patterns parallel to the corrugation 41 are formed by conventional photolithography. In this procedure the SiN.sub.x film 43 prevents the photoresist corrugation 41 from being dissolved into the developer. Using these striped patterns as a mask, etching of the SiN.sub.x film 43 is performed. In the region where the SiN.sub.x film is taken off, the InP substrate 42 is etched using the photoresist corrugation 41 as a mask to fabricate a diffraction grating on the InP substrate 42. Then the photoresist 44 covering the SiN.sub.x film 43 is removed. In the next step, an SiN.sub.x film corrugation 43, whose phase is reverse to the photoresist corrugation, is produced in the region where the SiN.sub.x film have been covered and not etched. In this process a unique characteristic of the ECR-CVD SiN.sub.x film 43 is utilised. The SiN.sub.x film 43 deposited on the photoresist 41 is etched by a buffered HF solution (BHF) at a higher rate than on the flat substrate. By optimising the etching condition, this difference in etching rates makes it possible to not etch only the parts of the SiN.sub.x film 43 on the flat substrates. After removing the photoresist and covering the previously formed grating on the substrate 42 with photoresist 45, the InP substrate 42 is etched using the SiN.sub.x film corrugation 43 as a mask. With this process, a quarter-wave-shifted diffraction grating is fabricated on the InP substrate 42. In this connection, see H. SUGIMOTO et al, "NOVEL FABRICATION METHOD OF QUARTER-WAVE-SHIFTED GRATINGS USING ECR-CVD SiN.sub.x FILMS", Electron. Lett., Nov. 19, 1987, Vol. 23, No. 24, pp. 1260-1261.
However, the above-discussed methods for fabricating a phase-shifted type diffraction grating have the following disadvantages:
In the first method (FIG. 1), while a fine pattern can be directly depicted on the substrate 2 with high accuracy, a proximity effect appears between adjacent gratings depending on the relationship between the grating interval (e.g., 0.25 .mu.m) and the electron beam diameter (e.g., 0.1 .mu.m) and a backscattering due to the electron beam scattering on a substrate surface having a photoresist coated thereon. As a result, it is difficult to form the diffraction grating 3 having a pitch of less than 0.3 .mu.m on a thick resist or a thick substrate. Further, its throughput and yield are inevitably reduced.
In the second method (FIG. 2), a complicated fabrication process is needed. That is, differences in appropriate exposure and development conditions for positive and negative photoresists 15 and 12 having different photosensitivities render the control of these conditions difficult. As a result, its reproducibility is poor and its yield is reduced.
In the third method (FIG. 3), the phase transient area 25 of the laser resonator 24 is widened due to a Fresnel diffraction of light beams 26a and 26b applied to cause the interference exposure unless the contact between the contact mask 21 and the photoresist layer coated on the substrate's surface 22 is sufficient.
In the fourth method (FIG. 4), the phase shifter 31 of a quartz plate having a step corresponding to a phase shift is disposed in the optical system, and this phase shifter 31 is irradiated with a slant projected light beam. The slant projection is required since the image must be formed on the sample surface 37 to which two beams are slantedly applied. As a result, aberrations occur on the sample surface 37, and hence an area on which the diffraction grating can be formed is limited.
In the fifth method (FIG. 5), it is difficult to control the etching conditions of the SiN.sub.x film 43 on the resist 41 and the substrate 42. Moreover, two etchings are performed for the substrate 42, and the control of the conditions is difficult. Thus, the shape of the diffraction grating is likely to be irregular, and the accuracies of depth and size thereof are lowered.